Isocyanate-derived polymers such as polyurethanes and polyureas have been workhorse materials in the automotive, furniture, carpet, construction, sporting goods, and packaging industries for many years. Production of these materials in foam, molding, casting, coating, and film forms presently exceeds two billion tons annually. Utilization of bis(isocyanates) as step growth monomers began in Germany in the 1930's and became industrially important primarily for the following reasons: 1) their derived polymers possessed unique and outstanding properties; 2) the synthesis of the isocyanate monomers by the phosgenation of aromatic amines was relatively inexpensive and conducive to large-scale manufacture; and 3) isocyanates react with alcohol and amine nucleophiles at high rates by nucleophilic addition in which no smaller molecule is generated that must be removed from the system.
Yet, despite their virtual ubiquitous application in our present day technology, some significant problems attend isocyanates and their polymers. An important problem is toxicity of both the isocyanates and amines derived from their hydrolysis and from their polymers. Isocyanate monomers have long been known to be exceedingly hyperallergenic, often causing permanent sensitization and serious health problems. Furthermore, the hydrolysis product of one of the most important poly(isocyanate) monomers, 2,4-toluenediisocyanate (TDI), is listed by the World Health Organization among the "chemicals which are probably carcinogenic for humans". This is reflected by the very low threshold limit value for most diisocyanates at 5 parts per billion on a time-weighted average over an 8 hour period. With the increased public consciousness about chemical pollution, this toxicity has led some European countries to presently prohibit the sale and use of new products containing poly(isocyanate) materials within their borders.
Aside from very significant toxicity issues, the high degree of reactivity of isocyanates has given rise to some practical problems in their application. Packaging of poly(isocyanates) must be scrupulously anhydrous, as water reacts to form unstable carbamic acids and finally amines. Reaction of these amines and residual isocyanate groups can then result in a prematurely crosslinked, useless material. Another problem is that the isocyanate groups can self-condense forming dimers and trimers. In the extreme these can result in undesirable crosslinking as discussed above, and even when occurring to a lesser extent, the self-condensation results in chain extension to an undefined extent. The precise isocyanate concentration is no longer known, and the stoichiometry which is critically important in step growth polymerizations is rendered a guessing game. A still further problem is that isocyanate groups are so reactive that they can engage in reaction with urethane and urea product linkages forming allophanate and biuret linkages, respectively. These reactions often result as well in undesirable, uncontrolled crosslinking.
Although epoxy-based systems have enjoyed relatively widespread industrial usage, nucleophilic additions to epoxies by alcohols and amines proceed at significantly slower rates than isocyanate systems. This is reflected in the conditions for cure of a typical two-package epoxy/amine structural adhesive requiring temperatures in excess of 15.degree. C. (60.degree. F.) for 18-24 hours. Furthermore, the epoxy systems are not without other problems and typically do not constitute an effective replacement for isocyanate-based polymers.
Azlactones (2-oxazolin-5-ones) have been known since the last century and would seem to offer features similar to isocyanates as well as some distinct advantages. The azlactone heterocycle reacts with nucleophiles by a similar addition reaction as the isocyanates resulting in ring-opened products. Furthermore, hydroxy and amine nucleophiles are reactive with azlactones, and the same commercial polyol and polyamine comonomer materials used with poly(isocyanates) can be used with poly(azlactones). As far as advantages, the azlactone offers significantly improved resistance to hydrolysis, i.e., ring-opening with water, and even if hydrolysis does occur due to prolonged exposure to moisture, the amidoacid hydrolysis products are not reactive with residual azlactones. Therefore, packaging and shelf stability of poly(azlactones) would not be the significant problems they are with poly(isocyanates). Another advantage of azlactone-based systems is in regard to uncontrolled crosslinking. The azlactones are not as highly reactive with nucleophiles as are especially the aromatic isocyanates which are the most common poly(isocyanates) employed. Consequently, control of reaction conditions and product linkages can be more easily achieved. Azlactones are not reactive with amide or ester product linkages and uncontrolled crosslinking does not occur in the fashion of isocyanate-urethane (allophanate) and -urea (biuret) reactions.
Utilization of bis(azlactones) as step-growth monomers was first disclosed in French Patent 887,530. Although the polymers were not well characterized, soluble, fiber-forming polyamides and polyesteramides were described, as was the use of bis(azlactones) as crosslinkers for cellulose acetate, poly(vinyl acetals), and casein. In a later more detailed study (J. Am. Chem. Soc., 77, 1541 (1955)), several bis(azlactones) were reacted with aliphatic diamines, aromatic diamines, and N-substituted aliphatic diamines. Solution polymerization provided semi-crystalline polyamides of variable molecular weight. Poly(2-imidazolin-5-ones) were provided by a variation of the reaction in which diamines containing basic groups or when certain added catalysts were utilized as disclosed in in U.S. Pat. No. 4,785,070. Heating at 180.degree.-200.degree. C. caused formation of catenary 2-imidazolin-5-one units via intrapolymeric cyclodehydration.
Bis(azlactones) have also been utilized as additives to hydroxy-functional polymers to increase performance in powder coatings (See U.S. Pat. No. 4,092,298) and in aromatic polyesters (See U.S. Pat. No. 4,291,152).
Bis(azlactones) which contain hydrogen substituents in the 4,4'-positions are mesionic and possess 1,3-dipolar character. This has been exploited to form polymers by cycloaddition reactions with electron deficient olefins as disclosed in U.S. Pat. Nos. 3,694,417 and 4,266,040.
Many of the applications of poly(isocyanates) require that materials be fluid at room temperature. What has probably contributed to the greatest extent to the more limited use of poly(azlactones) is that, with a few exceptions vide infra, the many reported bis(azlactones) are high melting solids. Table 1 of a review entitled "Polyazlactones" by J. K. Rasmussen, et al., Encyclopedia of Polymer Science and Engineering, Second Edition, Volume 11, 1988, pp.558-571 contains a listing of reported bis(azlactones) and their melting points and is incorporated herein by reference.
Two classes of azlactone-functional compounds that typically are fluids at room temperature are disclosed in U.S. Pat. Nos. 4,485,236 and 4,639,286. These two classes of azlactone compounds result from 1) the uncatalyzed Michael addition of highly nucleophilic secondary amine compounds to the .beta.-carbon of the alkenyl group of 2-alkenyl azlactones and 2) the acid-catalyzed Michael addition of thiol compounds to the same group of 2-alkenyl azlactones. These azlactone-functional reaction products differ compositionally and generally differ in method of preparation from the compositions and method of the present invention.